1. Field of the Invention
The present invention relates to a multilayer printed circuit board (PCB).
2. Related Background Art
Japanese Laid-Open Patent Publication No. 2000-188478 discloses a multilayer PCB in which the dimensions of conductors (signal vias) and a middle connecting layer (a via land) between the conductors are determined to stabilize the characteristic impedance of the PCB.
In one aspect, the invention relates to a multilayer PCB comprising: a first signal transmission line; a second signal transmission line opposite to the first transmission line; a first ground layer opposite to the first transmission line; a second ground layer opposite to the first ground layer; a first insulator disposed between the first and second transmission lines; a second insulator disposed between the first and second ground layers; a signal via passing through the first insulator and connected between the first and second transmission lines; and a ground via passing through the second insulator and connected between the first and second ground layers. The signal via is separated from the first and second ground layers. The ground via is separated from the signal via. The first ground layer has an end protruding with respect to the second ground layer. The end extends nearer to the signal via than an end of the second ground layer opposite to the end.
The end of the first ground layer may include a portion which abuts an end face of the ground via and a portion directly adjacent to the abutting portion. The end of the second ground layer may abut an opposite end face of the ground via. The adjacent portion of the first ground layer may extend from the ground via toward the signal via along the first transmission line beyond the end of the second ground layer.
An area of the first ground layer overlaid with the first transmission line enlarges due to the protruding end of the first ground layer. Therefore, the characteristic impedance of the transmission line is continuous by a longer length, and this improves impedance matching of the transmission line. Thus, the characteristic impedance of the PCB can be stabilized.
The end of the second ground is farer from the signal via than the end of the first ground layer Therefore, the ground via can be sufficiently separated from the signal via, and this can advantageously stabilize the characteristic impedance.
The PCB may further comprise another plurality of ground vias. The ground via and the other plurality of ground vias may be disposed at equal intervals on a line forming a square around the signal via. Such arrangement of the grand vias is beneficial when the first transmission line has two sides extending straight in parallel. In this case, some of the ground vias can be placed in parallel to these sides of the transmission line with a uniform distance. Therefore, it is possible to reduce the adverse effect of the ground vias on the impedance matching of the first transmission line. This can further stabilize the characteristic impedance.
The ground via and the other plurality of ground vias may be disposed at equal intervals on a line forming a circle around the signal via. The impedance matching can be favorably achieved by arranging the ground vias at equal intervals around the signal via. Such arrangement can simplify designing the PCB.
In another aspect, the present invention relates to a light transceiver comprising: the above PCB; and light-emitting module, a light-receiving module and an electronic element mounted on the PCB. The electronic element is electrically connected to the light-emitting module or the light-receiving module by the first and second transmission lines of the PCB.
In still another aspect, the present invention relates to a transponder comprising: the above PCB; a light-emitting module, a light-receiving module, a multiplexing IC and a demultiplexing IC fixed to the PCB. The light-emitting module may be electrically connected to the multiplexing IC by the first and second transmission lines of the PCB. The light-receiving module may be electrically connected to the demultiplexing IC by the first and second transmission lines of the PCB.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.